Method for determining a length of a trailer, driving system and vehicle

A method comprising: in response to determining that a vehicle 1 towing a trailer 2 is moving in a forward direction, capturing a first frame using a rear vehicle camera 4 at a first time an object 6 is detected in the first frame; capturing a second frame using a trailer camera at a second time when the when the object is first detected in the second frame; and determining the length of the trailer based on the speed of the vehicle and the time period between the first and second times. A frame comprising the object may be captured at by a front vehicle camera 3 at a third time, and the trailer length may be based on the time period between the third and first times. The method may be performed in response to determining the speed of the vehicle, or the curve radius of vehicle path to be above a threshold.